The Co-operation and Competition Panel (CCP) has ruled that PCTs can neither limit nor encourage referrals to specific providers as this would breach patients' right to choose where they are treated.
The ruling will have 'enormous consequences' on GPs' ability to control expenditure, said Dr Michael Dixon, chairman of the NHS Alliance.
'If you have consortia that have redesigned local services to save money, this means commissioners won't be able to ensure patients are referred to this service. You will end up with a very expensive system,' he said.
Under the CCP ruling, any process which 'directs patients to particular providers', or involves 'local price negotiations' or 'block contracts', breaches patients' right to choose where they are treated.
Dr Dixon said it meant GP commissioners could not stop patients being treated at expensive tertiary treatment centres, even if GPs feel the care will be poorer quality.
'With £20 billion to save GP commissioners must have the ability to control costs,' he said. Experts also believe that the savings available to PCTs and GP consortia from sharing back office functions are in the region of 'tens of millions', despite the DoH expecting GP consortia to save the NHS £1.4 billion in administration costs.
In October last year, NHS chief executive Sir David Nicholson said GP consortia could survive on 'significantly less' than PCTs' running costs by sharing back-office functions across 'bigger units'.
But John Neilson, managing director of NHS Shared Business Services, predicted around £30 million could be saved by PCTs if they all followed the best examples of back-office efficiency.